1. Field of the Invention
The present invention relates to a system for optimizing well production and more particularly, but not by way of limitation, to a down hole fluid level monitoring system coupled with a controlled variable speed pumping apparatus for maintaining a desired well bore fluid level to increase production.
2. History of the Art
The rate of flow of oil from a reservoir into the well bore is governed by numerous factors. Some of these are characteristic of the reservoir or of the method in which the well was completed, and as such, manipulation of the rate of flow is beyond the control of the well operator.
However, other factors change gradually over the life of the well and many require that the operator modify his production techniques and eventually begin secondary recovery procedures. One factor over which the operator has direct day to day control is the well bore fluid level. This variable has a direct relationship to the efficiency of well production, and the maintenance of this level at the proper height is one of the primary goals of the well operator.
The typical down hole configuration is that of a well casing which is perforated near the lower end thereof in communication with the oil reservoir. Concentric within this casing is the well tubing the lower end of which is in communication with the interior of the casing and which contains a pumping unit.
The pumping unit is operated by a reciprocal sucker rod contained within the tubing and during the pumping operation, the tubing above the pump and surrounding the sucker rod contains the oil which is being pumped to the surface. Stated another way, during pumping operations the tubing above the pump is generally full of oil to the surface. The annulus fluid level between the tubing and the casing may be anywhere from the surface down to the pump inlet itself. This annulus fluid level creates a back pressure at the perforations in the casing which is proportional to the density of the fluid multiplied by the height of the annulus fluid level. This back pressure reduces the effective pressure gradient from the reservoir to the well bore and thereby decreases the production of oil from the well. However, if the annulus fluid level can be kept just above the pump level the back pressure is reduced allowing more oil to flow in from the surrounding reservoir. When the annulus fluid level falls to the level of the pump or below, dry pumping occurs referred to as "fluid pounding" which can cause excessive damage to the pumping unit.
Therefore, it can be stated that when the fluid height is at the pump inlet level production is zero and as the fluid level rises above the pump level production sharply rises to its maximum value and then starts to fall off as the column height of the annulus fluid level increases thereby creating greater back pressure. When the back pressure exerted by the fluid level equals the static reservoir pressure, oil ceases to enter the well bore and production again becomes zero.
Clearly, maximum production occurs when the fluid level is just high enough to charge the pump but too low to create any significant back pressure. From a practical standpoint the goal of maintaining a consistently low fluid level while avoiding fluid pound has been an impossible one. Common approximations include intermittent production of the well and fluid pound detectors which shut the well in after it begins to pound. Neither of these approaches is completely satisfactory since they do not yield production and they do not eliminate shock and starting loads on pumping equipment.